U.S. President Donald Trump ignited a political firestorm on Tuesday by firing FBI Director James Comey, who had been leading an investigation into the Trump 2016 presidential campaign's possible collusion with Russia to influence the election outcome.

The Republican president said he fired Comey, the top U.S. law enforcement official, over his handling of an election-year email scandal involving then-Democratic presidential nominee Hillary Clinton.

The move stunned Washington and raised suspicions among Democrats and others that the White House was trying to blunt the FBI probe involving Russia.

Some Democrats compared Trump's move to the "Saturday Night Massacre" of 1973, in which President Richard Nixon fired an independent special prosecutor investigating the Watergate scandal.

White House officials denied allegations that there was any political motive in the move by Trump, who took office on Jan. 20.

Senate Democratic leader Chuck Schumer said he spoke to Trump and told him he was "making a very big mistake" in firing Comey, adding the president did not "really answer" in response.

An independent investigation into Moscow's role in the election "is now the only way to go to restore the American people’s faith," Schumer said.

Though many Democrats have criticized Comey's handling of the Clinton email probe, they said they were troubled by the timing of Trump's firing of him.

Senator Richard Burr, the Republican chairman of the Senate Intelligence Committee, which is overseeing its own investigation into Russian interference during the election, said in a statement he was also troubled by the timing of Comey's termination.

"His dismissal, I believe, is a loss for the Bureau and the nation," Burr said.